For a number of high-temperature applications, the use of composite materials formed from graphite and high-melting metals has gained acceptance in place materials comprised solely of high-melting metals. These composite materials are distinguished, in particular, by an improved heat storage capacity and a lower density.
The deciding factor for the usefulness of composite materials of graphite and high-melting metals is a good, high temperature-resistant joint between the parts of graphite and the high-melting metal. Moreover, adequate adhesion and long-term stability, as a result of the suppression of carbon diffusion from the graphite part into the metal part, are also equally important. Due to its good wetting properties and its comparatively low price, zirconium, having a melting point of about 1850.degree. C., has proven to be suitable in many applications as a solder for joining high-melting metals and graphite.
When a composite is produced from molybdenum or a molybdenum alloy and graphite, the zirconium solder normally forms a eutectic with the molybdenum. This eutectic typically has a melting point of about 1520.degree. C. For safety reasons, the temperature at which the composite is used should be at least 150.degree. C. below the melting point of the solder. Thus, the use of the composite is unfortunately restricted up to a temperature of about 1350.degree. C. For certain applications, this maximum use temperature is not sufficient.
DE-B 2,115,896, U.S. Pat. No. 4,597,095, DE-B 1,951,383 and DE-B 2,118,425 variously describe composite rotary X-ray tube anodes having a part consisting, for example, of molybdenum or a molybdenum alloy, which part is soldered to one or more parts of graphite. The solder material used is, for example, zirconium or a zirconium alloy. In addition, these references variously disclose interlayers arranged between the graphite and metal parts and the solder material.
In DE-B 2,115,896, no interlayers are provided between the parts to be soldered and the solder material itself, so that the above-mentioned problems with respect to the formation of a eutectic with a reduced melting point arise.
In U.S. Pat. No. 4,597,095, a layer of a non-carbide forming material, such as platinum, palladium or rhodium, is provided between the graphite part and the solder material. Although this interlayer prevents the formation of a zirconium carbide it does not prevent the formation of a eutectic between the molybdenum and zirconium. Thus, this approach suffers the above-mentioned disadvantages.
In DE-B 1,951,383, the soldering surfaces of the graphite parts are selectively provided with a layer of zirconium carbide, tantalum carbide or hafnium carbide before the soldering step. This embodiment, however, does not prevent the formation of a eutectic between molybdenum and zirconium.
In DE-B 2,118,425, an interlayer of tantalum and/or tungsten is provided between the molybdenum part and the solder. The thickness of this interlayer lies in the range between 0.1 and 1 mm, and it is pointed out that the limits indicated are not critical. This interlayer does prevent, to an extent, the formation of a eutectic between molybdenum and zirconium. However, the interlayer according to the reference is unable to relieve thermal stresses which arise during the soldering temperature because of the different thermal expansion of molybdenum and graphite. Thus, cracks can form in the solder material.
It is therefore an object of the present invention to provide a composite material of molybdenum or a molybdenum alloy and graphite, which is suitable for use with temperatures exceeding 1350.degree. C., but which also allows the use of zirconium as the solder material.